In electrophotographic equipment utilizing semiconductor-laser light for exposure, it is conceivable to reduce the spot diameter of laser light in order to obtain high image quality and high stability. In general, by shortening the wavelength of laser light, the spot diameter can be readily reduced. Infrared-laser light utilized in general laser printers is approximately 780 nm in wavelength, and red-laser light is approximately 650 nm in wavelength; therefore, the spot diameter can simply be reduced by 80%. Furthermore, blue-laser light is approximately 400 nm in wavelength, and therefore has as an advantage that the spot diameter can be reduced by approximately 50%.
Japanese Patent Application Laid-Open No. 2003-29217 (corresponding to U.S. Patent Application Publication No. US-2003-011865-A1) and Japanese Patent Application Laid-Open No. 2002-269792 propose circuits for driving semiconductor lasers.
However, in general, when the spot diameter of laser light is reduced, the forward voltage required for luminescence is higher than that in the case of a larger spot diameter. Accordingly, in order to reduce the spot diameter of laser light, a high-voltage-proof IC must be used and consequently productivity is reduced. In other words, one desired feature is to reduce the spot diameter of laser light while applying a low voltage to an IC.
For example, the forward voltage for a blue laser to emit light is 5.5 V and therefore higher than the power-source voltage (5 V) of general ICs. Accordingly, in order to drive an IC a blue laser, a high-voltage-proof IC must be used and consequently problems of cost arise. Moreover, although it is possible to conceive a drive circuit with discrete electronic components for which voltage can readily be raised, the problem of high-speed switching remains.